Kerberos: The Definitive Guide

Man-in-the-Middle Attacks

Finally, a man-in-the-middle attack affects most any protocol that attempts to verify the identity of connection endpoints (in this case, a user and the machine they wish to reach). A man-in-the-middle attack is an active attack , meaning that the attacker must be able to read all messages on the network as well as send out arbitrary messages of his own design.

The goal of the man-in-the-middle attack is to impersonate the server, resulting in the user thinking that he connected to the legitimate server, when in fact he is talking to the attacker. Once the attacker has control of the session, she can act as a simple pass-through (passing messages between the user and the legitimate server, without modification), or she can actively inject, modify, or delete messages between the user and the server. The attacker now is part of the conversation between the user and the legitimate server, and can modify any messages that pass through her, hence the name “man-in-the-middle.”

The good news is that the Kerberos protocol has built-in protection against man-in-the-middle attacks. Since Kerberos performs mutual authentication, by confirming not only the end user’s identity but also the server’s identity, man-in-the-middle attacks are thwarted.

To prevent against man-in-the-middle attacks, some mechanism to validate the server’s encryption key must exist. Other protocols use manual verification (such as Secure Shell, where the “fingerprint” of the server’s public ...